1. Field of the Invention
The invention relates to a catalytic converter that is housed within a pipe making up an exhaust gas emission system and is secured to the pipe.
2. Description of Related Art
A variety of efforts aimed at reducing environmental impacts and burden are being carried out on a global scale in many industrial fields. In the automotive industry, developments are constantly being made to expand the use and further enhance the performance of not only high fuel-performance gasoline engine vehicles, but also “eco cars” such as hybrid vehicles and electric cars. In addition to the development of such eco cars, active research is also being conducted on exhaust gas purifying catalysts which purify the exhaust gases emitted by engines. Such exhaust gas purifying catalysts include oxidation catalysts, three-way catalysts and nitrogen oxide (NOx) storage/reduction catalysts. In such exhaust gas purifying catalysts, the catalytic activity is exhibited by noble metal catalysts such as platinum (Pt), palladium (Pd) and rhodium (Rh). The noble metal catalysts are typically used in a state where the noble metal catalysts are supported on a support made of a porous oxide such as alumina (Al2O3).
Catalytic converters for purifying exhaust gases are generally installed in an exhaust system connecting a vehicle engine and a muffler. The engine may emit harmful substances to the environment, such as carbon monoxide (CO) and NOx, unburned hydrocarbons (HCs) and volatile organic compounds (VOCs). The catalytic converter is constructed so as to convert such harmful substances into acceptable substances. In a catalytic converter, a catalyst layer in which a noble metal catalyst including such as Rh, Pd, and Pt is supported on a support is disposed on a cell wall surface of a substrate. When exhaust gases are passed through the catalytic converter, CO is converted to CO2, NOx is converted to N2 and O2, and VOCs are burned, forming CO2 and H2O.
The support on which a noble metal catalyst is supported is exemplified by CeO2—ZrO2 solid solutions (commonly referred to as CZ materials or cerium oxide (ceria)-zirconia composite oxides). This is also called a promoter and is a constituent essential to the above-mentioned three-way catalyst which simultaneously removes the harmful substances CO, NOx and HCs in exhaust gases. CeO2 is one constituent essential to the promoter. CeO2, owing to the fact that the oxidation number changes between Ce3+ and Ce4+ depending on the partial pressure of oxygen within the exhaust gases to which CeO2 is exposed, has the function of absorbing and releasing oxygen to compensate for a surplus or deficiency of charge and has an oxygen storing function (which functions are collectively referred here as the oxygen storage capacity (OSC). Moreover, to ensure a conversion window for this three-way catalyst, the CeO2 absorbs and eases atmospheric fluctuations in the exhaust gases, enabling a close-to-stoichiometric air-fuel ratio to be maintained.
In catalytic converters, it is important how to lower the amount of noble metal catalysts used in such three-way catalysts in terms of reducing the “material risk” of, for example, rare metals and ensuring cost competitiveness. However, greatly decreasing the amount of noble metal catalysts in three-way catalysts also greatly lowers the catalytic activity, resulting in marked declines in, for example, the OSC, the low-temperature activity, and also the NOx conversion performance in a high-temperature environment. This is because a large decrease in the amount of noble metal catalysts greatly lowers the number of active sites, and the greatly lowered number of catalyst reaction sites leads to a pronounced decline in the conversion performance.
Of the noble metal catalysts Pt, Pd and Rh particularly used in three-way catalysts, Rh has the best NOx conversion performance. On the other hand, Rh has the highest market price per unit weight. Conventionally, a high OSC can be achieved by supporting Rh on a cerium oxide (ceria)-containing support. However, there is also a trade-off in that increasing the amount of cerium oxide in the support has the undesired effect of lowering the NOx conversion performance distinctive to Rh. Therefore, when Rh is used as a noble metal catalyst in a three-way catalyst, design guidelines are needed for creating three-way catalysts that optimize both the OSC and the NOx conversion performance.
Concerning the production of optimal three-way catalysts, in light of the differences in, performance between diverse noble metal catalysts and carriers depending on the constituents therein, intensive research is being conducted on zone-coated catalysts in which different constituents are arranged on the upstream and downstream sides of the substrate so as to be able to effectively elicit the characteristics of each constituent.
One such zone-coated catalyst is described in Japanese Patent Application Publication No. 2012-040547 (JP 2012-040547 A), which discloses an exhaust gas purifying catalyst including a substrate that forms gas flow channels for the flow of exhaust gases, and a catalyst layer formed on the substrate. More specifically, the catalyst layer employed here is made of a bottom catalyst layer, a first-stage top catalyst layer and a second-stage top catalyst layer. The bottom catalyst layer is formed on a surface of the substrate. The first-stage top catalyst layer covers a surface of the bottom catalyst layer on an upstream side in a gas flow direction. The second-stage top catalyst layer covers a surface of the bottom catalyst layer on a downstream side in the gas flow direction from the first-stage top catalyst layer. The bottom catalyst layer supports at least one of Pd and Pt, the second-stage top catalyst layer supports Rh, and the first-stage top catalyst layer supports Pd. The support which supports the Pd of the first-stage top catalyst layer is a Y2O3-containing ZrO2 composite oxide. It is described that, with this arrangement, the purifying properties of the catalytic noble metals can be fully exhibited, enabling the low-temperature conversion performance of the catalyst to be increased. It is also described that, by using a Y2O3-doped ZrO2 composite material having a low specific heat and excellent heat resistance as the support material in the first-stage top catalyst layer, the catalyst temperature rise properties are enhanced while also ensuring the heat resistance, thus enabling a durable catalyst warm-up performance to be obtained.
Japanese Patent Application Publication No. 2012-152702 (JP 2012-152702 A) discloses an exhaust, gas purifying catalyst having a substrate, a bottom catalyst layer which is formed on the substrate and includes at least one of Pd and Pt, and a top catalyst layer which is formed on the bottom catalyst layer and includes Rh. A region that does not include the top catalyst layer is provided on an exhaust gas upstream side of the exhaust gas purifying catalyst. The bottom catalyst layer is made of a first-stage bottom catalyst layer on an exhaust gas upstream side and a second-stage bottom catalyst layer on an exhaust gas downstream side. The first-stage bottom catalyst layer includes an oxygen storage material. It is described that this arrangement makes it possible to markedly suppress grain growth by the various catalyst metals supported in the respective catalyst layers, particularly the second-stage bottom catalyst layer on the exhaust gas downstream side and the top catalyst layer. Moreover, it is described that, by providing, on the exhaust gas upstream side, a region that does not include the top catalyst layer, the diffusibility of HCs to the interior of the first-stage bottom catalyst layer can be increased, thus promoting the conversion of HCs in the first-stage bottom catalyst layer and making it possible to achieve a satisfactory catalyst warm-up performance.
In addition, Japanese Patent Application Publication No. 2012-020276 (JP 2012-020276 A) discloses an exhaust gas purifying catalyst in which the catalyst layers making up the catalyst include a bottom catalyst layer, a first-stage top catalyst layer and a second-stage top catalyst layer. The bottom catalyst layer is formed on the surface of a substrate. The first-stage top catalyst layer covers the surface of the bottom catalyst layer on an upstream side in a gas flow direction. The second-stage top catalyst layer covers the surface of the bottom catalyst layer on a downstream side in the gas flow direction from the first-stage top catalyst layer. Here, the bottom catalyst layer supports at least one of Pd and Pt, the first-stage top catalyst layer supports Pd, and the second-stage top catalyst layer supports Rh. The first-stage top catalyst layer has a density of supported Pd of from 4.5 to 12 wt %. It is described that this arrangement enables the purifying properties of the noble metal catalyst to be fully exhibited, increasing the low-temperature conversion performance of the catalyst.
As described above, there exists a variety of art relating to zone-coated catalysts.